Macbook PRO RAM upgrade

I am about to buy 2x4GB RAM. But I heard something that old MBP became slower when users upgraded to 8GB of RAM and that only 6GB worked for them.

Now I wanna ask if this is the same case for the latest MBP 2010? If I upgrade to 8GB, will my computer become slower? Or better and faster?

THANK YOU in advance for your answer.

Nope the latest MBP can see and use all 8GB, depending on what you currently have installed, you may or may not see a speed increase, you will see an increase in performance you edit a lot of HD movies etc